Information on clouds, and in particular the height of cloud bases, is extremely important for many areas of meteorology, including aviation weather, relatively short term weather forecasts, and long-term climate studies. It is straightforward to measure the base height of a cloud from the ground. For example, many airports are equipped with ceiliometers for this purpose. In addition, data collected by radiosondes may be interpreted to estimate both cloud bases and tops, but with some drawbacks. For example, the ground-based measurements of cloud base height can be quite accurate, but radiosondes can become saturated in high humidity environments and provide inaccurate reading. In addition, their coverage is rather limited, i.e. to localized areas around an observation area. Thus, these measurements are obviously not available over the ocean, which covers 3/4 of the Earth's surface. Therefore, the retrieval of cloud base heights from global satellite observations becomes highly desirable.
There is a wide range of useful measurement accuracies for cloud base heights, depending upon the applications. For example, a minimum accuracy of 2 km is specified as a program requirement for the National Polar-orbiting Operational Environmental Satellite System (NPOESS). Such accuracy may prove useful for some climate studies. However, for airport approach and landing weather, especially for remote locations such as the approach over the Gulf of Alaska into Anchorage, and to develop global energy budgets used in climate change studies, higher accuracies, to the order of 30-100 m if possible, are required. There is a need for a satellite based approach for the determination of global cloud base information with such accuracies.
Satellite observation of clouds are made from several platforms. For example, the SSM/T-2 is a microwave radiometer flown on recently launched DMSP satellites and it is used primarily for the retrieval of water vapor profiles, as discussed by al Khalaf, "Retrieval of atmospheric water vapor profiles from the Special Sensor Microwave Temperature-2 (S SMT-2)." Ph.D. Dissertation, Department of Meteorology, Texas A&M University, College Station, Tex. 145, May 1995. In his work, al Khalaf treats liquid clouds explicitly in the retrieval which can be obtained from another DMSP sensor known as the Special Sensor Microwave Imager (SSMI). In so doing, al Khalaf's algorithms assign top and bottom heights to the cloud but does not retrieve them from the SSM/T-2 data.
A paper by T. T. Wilheit and K D. Hutchison "Water vapor profile retrievals from SSMT-2 data constrained by infrared-based cloud parameters," IEEE J. Remote Sensing, vol. 18 pp. 3263-3277, 1997 discloses that independent estimates of the cloud top height and phase can be used to further improve the water vapor profiles retrieved from SSM/T-2 data. They showed an example of a retrieval for a case where the radiosonde clearly showed the presence of a thin cloud layer. Without the use of cloud top information, which was retrieved from AVHRR imagery, the water vapor profile retrieval using SSM/T-2 data failed to detect the presence of the cloud. When the cloud top information was used to force a cloud in the retrieval of the microwave data, an "apparent" cloud thickness resulted. However, this research provided no indication on the reliability of the cloud thickness or the existence of a useful signature in the microwave data which could be exploited in the retrieval of a cloud base. Thus, while alluding to the "appearance" of a cloud base height in the paper, the authors noted that cloud base was considered totally unreliable and required additional research to determine if in fact the results were real or merely an artifact of the non-linear relationships in the retrieval algorithm.
Although much information on clouds is retrievable from satellite observation, there remains a need to identify a cloud base signature from satellite meteorological data. There is a further need to develop an algorithm to exploit the cloud base signature to determine cloud base heights for clouds around the globe and particularly over water where ground based observation is not available.